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Australian Earthquake Engineering Society 2012 Conference, Dec 7-9,
Tweed Heads
Living a New Era in Earthquake Engineering:
targeting damage-resisting solutions to meet societal
expectations
Stefano Pampanin
Associate Professor (Reader) in Structural Design and Earthquake Engineering
Department of Civil and natural Resources Engineering
University of Canterbury ,Christchurch, New Zealand
email: [email protected]
ABSTRACT
Earthquake Engineering is facing an extraordinary challenging era, the ultimate target being
set at increasingly higher levels by the demanding expectations of our modern society.
The Canterbury earthquakes sequence in 2010-2011 has confirmed a fundamental mismatch
between societal expectations over the reality of seismic performance of modern buildings.
By and large, with some unfortunate exceptions, modern multi-storey buildings performed as
expected from a technical point of view, considering the intensity of the shaking they were
subjected to.
In accordance to capacity design principles, plastic hinges developed in beams allowing for a
ductile beam-sway mechanism to develop and the building to stand. Nevertheless, in many
cases, these buildings were deemed too expensive to be repaired and were consequently
demolished.
Targeting life-safety is clearly not enough for our modern society and a paradigm shift
towards damage-control design philosophy and technologies is urgently required.
Is ductility-based design philosophy becoming obsolete and does it really imply irreparable
damage?
This paper will discuss motivations, issues and, more importantly, cost-effective engineering
solutions to design buildings capable of sustaining low-level of damage and thus limited
business interruption after a design level earthquake. Focus will be given to the extensive
research and developments in “jointed ductile” connections based on controlled rocking &
dissipating systems for reinforced concrete, steel and more recently (laminated) timber
structures.
An overview of recent on-site applications of low-damage or damage-control structural (and
non-structural) systems, featuring some of the latest technical solutions developed in the
laboratory and including proposals for the rebuild of Christchurch, will be provided as
successful examples of practical implementation of performance-based seismic design theory
and technology.
Keywords: performance-based design, low-damage structural systems, damage-control, posttensioning, PRESSS, Pres-Lam, Canterbury Earthquake
1
INTRODUCTION: THE 22 FEB 2011 EARTHQUAKE EVENT
The Mw 6.3 Christchurch (Lyttelton) earthquake (afterschok) occurred at 12.51pm on
Tuesday 22nd Feb 2011, approximately 5 months after the Mw 7.1 Darfield (Canterbury)
main shock (Fig. 1). The epicentre of the February event was approximately 10km south-east
of the Christchurch (Ōtautahi) Central Business District (CBD), near Lyttelton, at a depth of
approximately 5 km. Due to the proximity of the epicenter to the CBD, its shallow depth and
peculiar directionality effects (steep slope angle of the fault rupture), a significant shaking
was experienced in the city centre, the eastern suburbs, Lyttleton-Sumner-Porter Hills areas
resulting in 182 fatalities, the collapse of several unreinforced masonry buildings and of two
RC buildings, extensive damage often beyond reparability levels to several reinforced
concrete buildings, damage to tenths of thousands of timber houses and unprecedented
liquefaction effects in whole parts of the city.
Figure 1. . Left: Fault rupture length and aftershock sequence for the 4 Sept 2011, 22nd Feb 2011 13th
June 2011, 23 Dec 2011 earthquake events (source GNS) Science);
Right: 22 February event, impact on CBD
The combined effects of proximity, shallowness and directionality, led to a much greater
shaking intensity of the 22 Feb aftershock, as recorded in the City of Christchurch, than that
of the main shock on 4 Sept 2010. A wide range of medium-to- very high horizontal peak
ground accelerations, PGA, were recorded by the GeoNet Network in the CBD area, with
peaks exceeding 1.6g at Heathcove Valley and between 0.4g-0.7g in the CBD stations. This
variation confirms in general strong dependence on the distance from the epicentre (as typical
of attenuation relationships) but also on the site-specific soil characteristics and possibly
basin amplification effects. Notably, the recorded values of vertical peak ground
accelerations, in the range of 1.8-2.2g on the hills, were amongst the highest ever recorded
worldwide. In the CBD the highest value of peak ground vertical accelerations recorded were
in between 0.5g and 0.8g.
Figure 2 compares the elastic acceleration and displacement response spectra (5%-damped)
after the 22 Feb 2011 event, from four ground motions recorded in the Christchurch CBD
with the code-design level spectra (NZS1170:5, 2004 for 500-years and 2500-years return
period, Soil Class D and Christchurch PGA=0.22g). As it can be noted, the level of shaking
intensity, expressed in terms of spectral ordinates, that the buildings in the Centre Business
District were subject to, was very high, well beyond the 1/500 years event code-level design
when not (for a wide range of structural periods from 0.5s-1.75s) superior to the Maximum
Credible Earthquake level (MCE, 1/2500 years event).
N
EQ3:REHS
(S88E)
1.6
EQ1:CBGS
1.4
Mean of 4
CBD records
1.2
1.0
0.8
EQ1:CBGS (NS64E)
EQ2:CHHC (S89W)
0.6
0.4
EQ4:CCCC
EQ2:CHHC
EQ4:CCCC (N89W)
NZS1170:5 (2004)
500−
500−year motion
NZS1170:5 (2004)
2500−
2500−year motion
0.2
0.0
NZS4203 (1976)
0
0.5
1
1.5
2 2.5 3
Period (sec)
N
1200
EQ3:REHS
Principal direction
Spectral Displacement / S d (mm) .
Spectra Acceleration / S a (g ms-2) .
1.8
EQ3:REHS
1000
EQ1:CBGS
800
EQ2:CHHC
600
4
4.5
5
Mean of 4
CBD records
NZS1170:5
(2004) 2500−
2500−
year motion
EQ4:CCCC
EQ2:CHHC (S89W)
NZS1170:5 (2004)
500−
500−year motion
400
200
EQ1:CBGS (NS64E)
Principal direction
NZS4203 (1976)
0
3.5
EQ3:REHS
(S88E)
EQ4:CCCC
(N89W)
0
0.5
1
1.5
2
2.5
3
Period (sec)
3.5
4
4.5
5
Figure 2. Acceleration and Displacement response spectra from 22 Feb 2011 Christchurch Earthquake
records compared with code design spectra (NZS1170:5)
An overview on the level of shaking and overall structural performance of buildings in the 4
Sept 2010 and 22 Feb 2011 earthquakes events can be found in Kam et al. (2010) and Kam
and Pampanin, (2011). For more comprehensive information on the overall earthquake
impact, the reader is referred to the two Special Issues of the Bulletin of the New Zealand
Society for Earthquake Engineering related to the 4 Sept 2010 and 22 Feb 2011 events
(NZSEE, 2010, 2011).
Observed building damage
Considering the high level of shaking, which led to high inelastic behaviour and severe
displacement/deformation demands, the overall behaviour of modern reinforced concrete
structures (dominant type of multi-storey building in the CBD) can be classified in general as
quite satisfactory.
However, the extent of structural damage in the plastic hinge regions, intended to act as fuses
as part of the ductile sway mechanism, highlighted the whole controversy of traditional
design philosophies, mainly focused on collapse-prevention and life-safety and not yet
embracing a damage-control objective. Figure 3 shows a examples of the extent of structural
damage in frames and shear walls in reinforced concrete multi-storey buildings (typically
precast with emulation of cast-in situ connections).
Figure 3. Damage to post-1980s RC moment-resisting frames and walls
Such post-earthquake damage situation and the following decision to demolish and rebuild instead of
repairing and strengthen was the most common scenario for the vast majority of reinforced concrete
multi-storey buildings in the CDB.
A summary and breakdown of the of the placard key statistics from the processed Building
Safety Evaluation (Post-earthquake inspection) database according to the type of structural
system and year of construction can be found in (Kam et al., 2011, Kam and Pampanin, 2011,
Pampanin et al., 2012)
In general whilst when referring to pre-1970s buildings (most of which had not been
seismically strengthened) their relatively poor performance did not come as a surprise (nearly
48% of pre-1970s buildings were assigned yellow or red tagged and the collapse of one
1960s RC building led to multiple fatalities, Kam et al, 2011), the high number of modern
buildings (at least post-1976, or post-1980s, thus designed in accordance with the basic
principles of capacity design) to be demolished represents a serious concern and a wake up
for the international earthquake community. Approximately 30% of the RC buildings in this
class were yellow or red tagged (Kam et al., 2011). The collapse of one 1980s RC building,
the Canterbury Television Building, or CTV, caused the highest number of fatalities
(Canterbury Earthquake Royal Commission of Enquiry, CERC, 2012).
As a consequence of the excessive cost-of-repairing (a well as, to some extent, of the
possibility to rely upon a significant insurance coverage for partial or full replacement of the
building) many relatively modern buildings (mid-1980s and onwards) have been or are being
demolished (Fig. 4 left, recent aerial view of the CBD). On a positive note, if any, this
massive man-made demolition is providing the opportunity for a significant re-design of the
urban plan of the city as part of the rebuild (Fig. 4 right).
Figure 4. Top Left: aerial view of the CBD at November 2012 and recently proposed urban plan for the
city centre reconstruction (CERA Blueprint)
2
REALITY CHECK: IS CURRENT DESIGN PHILOSOPHY MEETING
SOCIETAL EXPECTATIONS?
Ductility and damage: is this an unavoidable equivalency?
The 22nd Feb 2011 Earthquake in Christchurch, New Zealand, has indeed and once more
highlighted the severe mismatch between the expectations of building occupants and owners
over the reality of the seismic performance of engineered building
A question is being raised: is ductility-based design philosophy becoming obsolete and does
it really imply irreparable damage?
As well known within the earthquake engineering community, but apparently not clear at all
in the wider public arena, the basic principle of the current seismic design philosophy of
ductile structures, referred to as “capacity design” or hierarchy of strength, developed in the
mid/late1960s by Professors Bob Park and Tom Paulay at the University of Canterbury in
New Zealand, is to ensure that the “weakest link of the chain” within the structural system is
located where the designer wants and will behave as a ductile “fuse”, protecting the structure
from more undesired brittle failure mechanisms (Fig. 5).
The inelastic action is intentionally concentrated within selected discrete “sacrificial” regions
of the structure, typical referred to as plastic hinges.
Until recent years, the development of inelastic action in traditional monolithic (or emulative)
ductile connections has been assumed to inevitably lead to structural damage, thus implying
that “ductility= damage”, with associated repair costs and business downtime.
Figure 5: A tribute to the basic concept of capacity design: the “weakest link of the chain” concept (left)
and its implementation in a frame system with the protection of a soft-storey (brittle) mechanism in
favour of a beam side-sway (ductile) mechanism (Paulay and Priestley, 1992).
As later discussed in this paper, following the recent development of alternative cost-efficient
solutions for high-performance low-damage structural systems, such ductility-damage
equivalency is not anymore to be considered an unavoidable compromise of a ductile design.
What is the acceptable level of damage?
According to current performance-based design approach, visually summarized with the
Performance Design Objective Matrix in Figure 6, different (and often not negligible) levels
of structural damage and, consequently, repairing costs shall thus be expected and, depending
on the seismic intensity, be typically accepted as unavoidable result of the inelastic
behaviour. Performance levels are expression of the maximum acceptable extent of damage
under a given level of seismic ground motion, thus representing losses and repair costs due to
both structural and non-structural damage.
To give a practical example, according to the Basic Objective presented in this performance
matrix, and associated to ordinary residential/commercial construction, a Life Safety damage
level would be considered acceptable under a design level earthquake (traditionally taken as a
500 years return period event). This would imply that extensive damage, often beyond the
reparability threshold (corresponding to a yellow/orange to red tag of the building), would be
considered as an accepted/proposed target.
Repairable
Irreparable
Figure 6. Seismic Performance Design Objective Matrix as defined by SEAOC Vision 2000 PBSE
Guidelines, herein rearranged to match building tagging, and proposed/required modification of the
Basic-Objective curve towards a damage-control approach (blue line, modified after Pampanin, 2010,
Kam et al., 2011)
Such implications are clear and obvious to the technical professionals, but not necessary to
the general public.
It should thus not come as a surprise if users, residents, clients, owners/stakeholders of the
building/facilities as well as the territorial authorities had (not only in the occasion of
Canterbury Earthquake, but also in similar situations in the past) a remarkably different
opinion, based on a clearly different understanding of the significance of, and expectation
from the behaviour of, an “earthquake-proof” building.
From the public perspective, not only life-safety and collapse prevention should be
considered as “granted”, but also only a minimum level of damage should be actually
expected so to require minimum repairing costs and disruption of the daily activities.
The renewed challenged of earthquake engineering: raising the bar to meet societal
expectation
In order to resolve this major perception gap and dangerous misunderstanding, a twofold
approach is required:
•
On one hand, increase the level of communication between scientists/researchers,
practitioner engineers, territorial authorities, Industry representatives and/or, generally
speaking, end-users. Define, set, agree and disclose to the wide public the
accepted/targeted performance levels built in a Building Act or in a design code,
including the not-written considerations and compromise between socio-economical
consequences and technical limitations and costs. It shall be clear that these are to be
considered “minimum”, not “maximum” or target, standards, with the possibility, and
somehow ethical duty, of achieving better performance if required/desired and when
practically feasibile.
•
On the other hand, significantly “raise the bar” by shifting the targeted performance
goals from the typically accepted Collapse Prevention or Life-Safety level, to a more
appropriate and needed Damage-Control level. This could be represented within the
Performance Objective Matrix by a tangible shift of the Objective Curves to the left,
i.e. towards higher performance levels or, equivalently, lower acceptable damage
levels (Fig. 6, dashed line).
Moreover, the focus of the next generation of performance-based design framework should
more explicitly directed towards the development of design tools and technical solutions for
engineers and stakeholders to control the performance/damage of the building system as a
whole, thus including superstructure, foundation systems and non-structural elements.
Valuable tentative recommendations/suggestions have been proposed in the past in terms of
pair of limit states or performance requirements for both structural (the “skeleton”) and nonstructural elements (the “dress”). Yet, practical cost-efficient solutions for low-damage
resisting non-structural elements daily use of practitioners and contractor need to be specified
and developed.
Not unexpectedly, the sequence of strong aftershocks that followed the main 4 September
2010 event, caused significant and repetitive damage to the non-structural components,
requiring continuous and expensive repairing. Work is in progress in this space with the clear
target to address this next fundamental step towards the development of an ultimate seismic
resisting system as the society expects (Baird et al., 2011; Tasligedik, 2012).
Furthermore, the Canterbury earthquake has emphasised the actual impact of having a
combined damage in the superstructures and in the foundation-soil system (Giorgini et al.,
2011). The area of Soil-Foundation-Structure Interaction has received in the past decades a
substantial attention reaching a significant maturity. Yet, there is strong need to convert the
available information into practical guidelines for an integrated structure-soil-foundation
performance based design.
This would require the definition and setting of specific and jointed limit states for the
superstructure and the foundation and suggest the corresponding design parameters to
achieve that “integrated” level of performance (Millen et al 2013). In the first phase of the
Christchurch Rebuild, this issue is becoming more apparent, as the designers of new
buildings are requested by the clients to be able to specify the targeted performance of the
building property as a whole, thus including the superstructure (structural skeleton and nonstructural elements) and foundation-soil system.
3
RAISING THE BAR: THE NEXT GENERATION OF DAMAGE-RESISTING
SYSTEMS
The breakthrough of jointed ductile “articulated” systems: PRESSS-technology
A revolutionary alternative technological solution for precast concrete connections and
system, capable of achieving high-performance (low-damage) at comparable costs has been
introduced in the late 1990s as main outcome of the U.S. PRESSS (PREcast Seismic
Structural System) program coordinated by the University of California, San Diego
(Priestley, 1991, 1996; Priestley et al. 1999) and culminated with the pseudo-dynamic test of
a large scale Five Storey Test Building (Fig. 7 left).
Figure 7. Left: Five-Storey PRESSS Building tested at University of California, San Diego (Priestley et al.,
1999); Right: Jointed precast “hybrid” frame and wall systems (fib, 2003; NZS3101:2006)
In PRESSS frame or wall systems, moment-resisting dry jointed ductile connections are
obtained by connecting precast elements through unbonded post-tensioning tendons/strands
or bars. In the so-called hybrid system (Priestley et al., 1996; Stanton et al. 1997, Fig.7
right), unbonded post-tensioned bars or tendons are combined with non-prestressed mild steel
(or similarly additional external dissipation devices as discussed in the next sections),
inserted in corrugated metallic ducts and grouted to achieve fully bond conditions.
During the earthquake shaking, the inelastic demand is accommodated within the connection
itself (beam-column, column to foundation or wall-to-foundation critical interface), through
the opening and closing of an existing gap (rocking motion). The mechanism acts as a fuse or
“internal isolation system” with negligible or no damage accumulating in the structural
elements, which are basically maintained in the elastic range. The structural skeleton of the
building would thus remain practically undamaged after a major design level earthquake
without any need for structural repairing intervention.
This is a major difference and improvement when compared to cast-in-situ solutions where,
as mentioned, damage has to be expected and it is actually accepted to occur in the plastic
hinge regions, leading to substantial costs of repairing and business interruption. The
traditional plastic hinge, or sacrificial damage-mechanism, is thus substituted by a sort of
“controlled rocking” (dissipative and re-centering) at the critical interface with no or
negligible damage (Fig. 8 left).
Moreover, the tendons are unbonded so to elongate within the duct without yielding. They
can thus act as re-centering “springs”, guaranteeing that the structure come back to its
original at-rest position at the end of the shaking This re-centering & dissipating mechanism
is described by a peculiar “flag-shape” hysteresis behaviour, whose properties and shape can
be modified by the designer by varying the (moment) contributions, between the re-centering
and the dissipation components (Fig. 8 right).
Figure 8. Left: Comparative response of a traditional monolithic system (damage in the plastic hinge and
residual deformations) and a jointed precast (hybrid) solution (rocking mechanism with negligible
damage and negligible residual deformations, fib, 2003); Right; Flag-shape hysteresis loop depending on
the balance between re-centering and dissipative contribution.
One step further: reparability of the weakest link of the chain: “Plug&Play”
replaceable dissipaters
In principle, either internal (grouted) mild steel bars or, more recently developed, external &
replaceable supplemental damping devices can be adopted (Fig. 9). The original solution for
hybrid connections proposed in the U.S.- PRESSS Program relied upon the use of grouted
mild steel rebars, inserted in corrugated (metallic) ducts. A small unbonded length in the mild
steel bars is typically adopted at the connection interface to limit the strain demand in the
reinforcing bars and protect them from premature rupturing when the gap opens up to the
design level of drift.
Figure 9. Left: Internal versus external replaceable dissipaters/fuses at the base-column/pier connection
(Marriott et al. 2008); Right: stable hysteresis loop of a typical dissipater
A potential downside of such an approach is that, following an earthquake, the internal rebars
would not be easily accessible nor replaceable as per a typical monolithic solution. Also the
degradation of bond between concrete and steel during reversal cyclic loading causes some
level of stiffness degradation thus potentially higher level of deformability of the structure.
More recently, following the declared target to achieve a low- (or no-) damage system,
significant effort has been dedicated in the past few years towards the development of costefficient external dissipaters, referred to as “Plug&Play” (PnP), for their capability to be
easily mounted and if required, demounted and replaced after an earthquake event
(Pampanin, 2005). This option would give the possibility to conceive a modular system with
replaceable sacrificial fuses at the rocking connection, acting as the “weakest link of the
chain”, according to capacity design principles.
One of the most efficient and practical PnP dissipater solution, developed and tested as part
of several subassembly configurations, consist of axial, tension-compression yielding mild
steel short-bar-elements, machined down to the desired “fuse” dimension and inserted and
grouted (or epoxied) in a steel tube acting as anti-buckling restrainers.
A number of tests have been successfully carried out at the University of Canterbury in the
past ten years on different subassemblies configuration including beam-column joint
connections, wall systems, column (or bridge pier)-to-foundation connections (Fig. 10) with
the aim to further simplify the constructability/assemblage and improve the reparability of the
structure after an earthquake event, thus dramatically reducing the costs associated to the
direct repairing of the structural system and to the downtime (business interruption).
Top Displacement (mm)
Lateral Force (kN)
-100 -80 -60 -40 -20
20
0
20
40
60
80 100
15
10
5
0
-5
HJ3-25PT1-7D
HJ3-27PT2-8D
-10
-15
-20
-5
-4
-3
-2
-1
0
1
2
3
4
5
Top Drift (%)
Figure 10. Alternative configurations of external replaceable (PnP) dissipaters for hybrid systems: Top
left and centre: beam-column connections, with and without recess in the beam (from Pampanin et al.
2006); Top right: Column to foundation connections (from Marriott et al., 2009) Bottom: typical flagshape hysteresis loops for a hybrid beam-column joint and a column-to-foundation connection with
external dissipaters.
A second generation of self-centering/dissipative high-performance systems, referred to as
Advanced Flag-Shape systems (AFS) has been recently proposed by Kam et al., 2010.
AFS systems combine alternative forms of displacement-proportional and velocityproportional energy dissipation (i.e. yielding, friction or viscous damping) in series and/or in
parallel with the main source of re-centering capacity (given by unbonded post-tensioned
tendons, mechanical springs or Shape Memory Alloys, SMA, with super-elastic behaviour).
As a result, it is possible to achieve an enhanced and very robust seismic performance, under
either far field or near field events (high velocity pulse), as proven by numerical
investigations (Kam et al., 2010) as well as shake table testing (Marriot et al., 2008).
4
EXTENSION TO MULTI-STOREY TIMBER BUILDINGS: THE PRES-LAM
SYSTEM
The concept of post-tensioned hybrid (recentering/dissipating) systems has been recently and
successfully extended from precast concrete to timber frames and walls (Palermo et al., 2005,
2006, Pampanin et al., 2006), in what is referred to as Pres-Lam (Prestressed Laminated
timber) system. Since 2004, a series of experimental tests (comprising quasi-static cyclic,
pseudodynamic and shake-table), have been carried out on several subassemblies or larger
scale systems at the University of Canterbury to develop different arrangements of
connections for unbonded post-tensioned timber frame and walls (Figs. 11-13).
HYBRID OR CONTROLLED ROCKING SYSTEM
Internal dissipation
devices
Rocking motion
θimp
Unbonded posttensioned tendon
INTERNAL DISSIPATERS:
epoxied mild steel bars with unbonded length
EXTERNAL DISSIPATERS:
mild steel rods with epoxied encased steel tubes
50 mm
(a) Internal and external dissipaters and construction details.
Hybrid specimen 1 - HY1
20
15
10
5
0
-5
-10
-15
fp0 = 0.8fpy
-20
-0,05 -0,04 -0,03 -0,02 -0,01
Hybrid specimen 3 – HY3
15
Top-lateral Force [kN]
T op-lat eral Force [kN]
20
10
5
0
-5
-10
-15
fp0 = 0.6fpy
-20
0
Drift
0,01
0,02
0,03
0,04
0,05
-0,05 -0,04 -0,03 -0,02 -0,01
0
Drift
0,01
0,02
0,03
0,04
0,05
(b) Force-drift relationships for several different joints with internal and external dissipaters.
Figure 11. Arrangements and testing results of Pres-Lam beam-column joints with internal or external
reinforcement (Palermo et al., 2005, 2006)
Figure 12. Testing of hybrid post-tensioned column-to-foundation connections with replaceable
dissipaters (observed performance at 4.5% drift) (Palermo et al., 2006)
Figure 13. Left and Centre: Pres-Lam coupled walls with U-shape Flexural Plates dissipaters (Iqbal et al.,
2007); Right: shake table test on Advanced-Flag-Shape Pres-Lam wall (viscous and hysteretic dampers in
parallel), Marriott et al., 2008)
Due to its high homogeneity and good mechanical properties, Laminated Veneer Lumber
(LVL) has been selected as the preferred engineered wood material for the first phase of the
research and development. Any other engineered wood product as Glulam or Cross-lam (Xlam) can be adopted and in fact research is more recently on-going using both of them in
addition to LVL.
The experimental results provided very satisfactory results and confirmation of the high
potential of this new construction system, referred to as Pres-Lam, which gives opportunities
for much greater use of timber and engineered wood products in large buildings, using
innovative technologies for creating high quality buildings with large open spaces, excellent
living and working environments, and resistance to hazards such as earthquakes, fires and
extreme weather events (Buchanan et al., 2009).
A major multi-year R&D project has been ongoing from 2008-2013 under the umbrella of a
NZ-Australia Research Consortium, STIC Ltd (Structural Timber Innovation Company).
5
ON-SITE
IMPLEMENTATIONS
TECHNOLOGY IN NEW ZEALAND
OF
PRESSS
AND
PRES-LAM
The continuous and rapid development of jointed ductile connections using PRESSStechnology for seismic resisting systems has resulted, within a bit more than one decade, in a
wide range of alternative arrangements currently available to designers and contractors for
practical applications, and to be selected on a case-by-case basis (following cost-benefit
analysis). An overview of such developments, design criteria and examples of
implementations have been given in Pampanin et al., (2005) and more recently in the
PRESSS Design Handbook (2010)
Several on site applications of PRESSS-technology buildings have been implemented in
different seismic-prone countries around the world, including, but not limited to, U.S.,
Central and South America, Europe and New Zealand. One of the first and most glamorous
application of hybrid systems in high seismic regions was given by the Paramount Building
in San Francisco (Fig. 14), consisting of a 39-storey apartment building and representing the
highest precast concrete structure in a high seismic zone (Englerkirk, 2002). Perimeter
seismic resisting frames were used in both directions. The dissipation was provided by
internally grouted mild steel with a short unbonded length at the critical section interface to
prevent premature fracture of the rebars.
Figure 14. Paramount Building, 39-storey building, San Francisco (Englerkirk, 2002, photos courtesy of
Pankow Builders, E. Miranda, Len McSaveney).
Given the evident structural efficiency and cost-effectiveness of these systems (e.g. high
speed of erection) as well as flexibility in the architectural features (typical of precast
concrete), several applications have quickly followed in Italy, through the implementation of
the “Brooklyn System” (Fig. 15), developed by BS Italia, Bergamo, Italy, with draped
tendons for longer spans and a hidden steel corbel (Pampanin et al., 2004). Several buildings,
up to six storeys, have been designed and constructed in regions of low seismicity (gravityload dominated frames). These buildings have different uses (commercial, exposition,
industrial, hospital), plan configurations, and floor spans. A, overview on the system can be
found in Pampanin et al. (2004).
Figure 15. Application in Italy of the Brooklyn System, B.S. Italia, with draped tendons (Pampanin et al.,
2004).
The first multi-storey PRESSS-building in New Zealand is the Alan MacDiarmid Building at
Victoria University of Wellington (Fig. 16), designed by Dunning Thornton Consulting Ltd.
The building has post-tensioned seismic frames in one direction and coupled post-tensioned
walls in the other direction, with straight unbonded post-tensioned tendons.
Figure 16. First multi-storey PRESSS-Building in New Zealand (Structural Engineers: Dunning
Thornton Consultants; Cattanach and Pampanin, 2008).
This building features some of the latest technical solutions previously described, such as the
external replaceable PnP dissipaters in the moment-resisting frame and unbonded posttensioned sandwich walls coupled by slender coupling beams yielding in flexure. Additional
novelty was the use of a deep cap-beam to guarantee rocking of the walls at both the base and
the top sections (Cattanach and Pampanin, 2008). This building was awarded the NZ
Concrete Society’s Supreme Award in 2009 and several other innovation awards.
The design and construction of the second PRESSS-Building in NZ and first in South Island
has followed at close distance and is represented by the Endoscopy Consultants’ Building in
Christchurch, designed for Southern Cross Hospitals Ltd by Structex Metro Ltd (Fig. 17).
Also in this case both frames and coupled walls have been used in the two orthogonal
directions. The post-tensioned frame system relies upon an asymmetric section reinforcement
with internal mild steel located on the top of the beam only and casted on site along with the
floor topping. The unbonded post- tensioned walls are coupled by using U-Shape Flexural
Plates solutions.
Figure 17. Southern Cross Hospital, Christchurch Rendering, construction of the frame, details of
beams, walls and UFP dissipaters (Structural Engineers: Structex; Pampanin et al., 2011).
It is worth noting that both these later structures have been designed and modelled, during the
design and peer review process, following the theory and step-by-step procedures included in
in the PRESSS Design Handbook (2010), published by the NZ Concrete Society, which
provides a full design example of a five-storey building in accordance to the NZS3101:2006
concrete design code Appendix B.
Real earthquake testing: when reality meets expectations
The Southern Cross hospital endoscopy Building has very satisfactorily passed the very
severe tests of the recent Christchurch earthquake series. In particular, the 22 February
earthquake was very close to the hospital with a very high level of shaking. Figure 18 shows
the minor/cosmetic level of damage sustained by the structural systems which comprise posttensioned hybrid frames in one directions and post-tensioned hybrid walls coupled with Ushape Flexural Plate Dissipaters. Important to note, the medical theatres with very
sophisticated and expensive machinery were basically operational the day after the
earthquake. One of the main feature in the design of a rocking-dissipative solution is in fact
the possibility to tune the level of floor accelerations (not only drift) to protect both structural
and non-structural elements including content and acceleration-sensitive equipment.
More information on the design concept and performance criteria, modelling and analysis,
construction and observed behaviour of the building can be found in Pampanin et al., (2011).
Figure 18. Negligible damage, to both structural and non-structural components, in the Southern Cross
Hospital after the earthquake of 22 February.
Implementation of Pres-Lam Buildings
Following the aforementioned extensive research and development on post-tensioned timber
buildings at the University of Canterbury, the first world-wide applications of the Pres-Lam
technology are occurring in New Zealand. Several new buildings have been constructed
incorporating Pres-lam technology.
The world’s first commercial building using this technology is the NMIT (Nelson,
Marlborough Institute of Technology) building, constructed in Nelson. The building has
vertically post-tensioned timber walls resisting all lateral loads as shown in Figure 19
(Devereux et al., 2011). Coupled walls in both direction are post-tensioned to the foundation
through high strength bars with a cavity allocated for the bar couplers. Steel UFP devices link
the pairs of structural walls together and provide dissipative capacity to the system. The
building was opened in January 2011.
Figure 19. The World’s first Pres-Lam building implementing unbonded post-tensioned
rocking/dissipative timber walls. Nelson Marlborough Institute of Technology, (NMIT), New Zealand
(Structural Engineers Aurecon, Devereux et al., 2011, Architects Irving-Smith-Jack)
The Carterton Events Centre (Fig. 20), located 100km north of Wellington, is the second
building in the world to adopt the Pres-Lam concept. Post-tensioned rocking walls were
designed as the lateral load resisting system (six walls in one direction and five in the other
direction). The post-tensioning details are similar to the NMIT building, while internal
epoxied internal bars are used for energy dissipation (Figure 20 right).
Figure 20 Carterton Events Centre. Single-storey building with LVL truss roof (Designed by Opus
International: Dekker et al. 2012).
The University of Canterbury EXPAN building (Fig. 21) was originally a two-third scale
prototype building tested in the laboratory under severe bi-directional loading conditions
(Newcombe et al., 2010). After a successful testing programme, the building was dismantled
and re-erected as the head office for the Research Consortium STIC (Structural Timber
Innovation Company Ltd). Due to the low mass, the connections were redesigned from
hybrid to purely post-tensioned without any dissipation devices. The light weight of the
structure allowed the main timber frames of the building to be post-tensioned on the ground
and lifted into places shown in Figure 21.
Figure 21. From laboratory specimen to office building: 3D Test Specimen tested in the lab (Newcombe et
al, 2010), demounted and reconstructed (Smith et al., 2011) on UC campus as EXPAN/STIC office
The new College of Creative Arts (CoCa) building for Massey University’s Wellington
campus has been recently completed (Fig. 22). The building is the first to combine posttensioned timber frame with innovative draped post-tensioning profiles to reduce deflections
under vertical loading. Additional dissipation is added in the frame directions by using UShape Flexural plate devices, placed horizontally and activated by the relative movement
between (some of) the first floor beams and elevated concrete walls/pedestal. This is a mixed
material damage-resistant building which relies on rocking precast concrete PRESSS walls in
one direction and Pres-Lam timber frames in the other direction.
Figure 22 College of Creating Arts (CoCa) Building, Massey University, Wellington, New Zealand
(Structural Engineers: Dunning Thornton Consultants)
As part of the Christchurch Rebuild, a number of buildings under construction or design will
implement the aforementioned damage-resisting technologies (Figs. 23-25), in some cases
using mixed materials and/or a combination with base isolations and other supplemental
damping devices.
Figure 23. Christchurch Rebuild: First Pres-Lam building in Christchurch, Merritt Building, Structural
Engineers: Kirk and Roberts; Architects: Sheppard and Rout
Figure 24. Christchurch Rebuild. Left: St Elmo Courts a 1930 RC building demolished Right :rendering
of the “new St. Elmo” using a combination of base-isolation and a post-tensioned timber-concrete twoway frame in the superstructures, Architect: Ricky Proko, Structural Engineers: Ruamoko Solutions;
Figure 25. Christchurch Rebuild: Trimble Building, Architecture and Structures from Opus
International (Brown et al.., 2012)
6
ACKNOWLEDGEMENTS
The research, development and implementation of damage-control solutions described in this
paper are the results of the exceptional support and collaborative effort between a number of
individuals and organizations from academia, the wider industry, governmental and funding
agencies, at national and international level, a list of whom would be practically impossible to
prepare. The author wishes to acknowledge and sincerely thank all those involved in this
extended research team.
67
REFERENCES
Buchanan, A.H., Palermo, A., Carradine, D., Pampanin, S., (2011), “Post-tensioned Timber
Frame Buildings” The Structural Engineer, Institute of Structural Engineers,(IStructE), UK,
Vol 89(17), pp. 24-31, ISSN: 1466-5123
CERC (2012) Canterbury Earthquake Royal Commission websites. Canterbury Earthquake
Royal Commission (CERC) website. Available at: http://canterbury.royalcommission.govt.nz
Cattanach A., and Pampanin, S. (2008) “21st Century Precast: the Detailing and Manufacture
of NZ's First Multi-Storey PRESSS-Building”, NZ Concrete Industry Conference, Rotorua
Dekker, D., Chung, S. and Palermo, A. (2012) ”Carterton Events Centre Auditorium PresLam Wall Design and Construction”. New Zealand Society for Earthquake Engineering
(NZSEE) Conference, Christchurch 13-15 Apr 2012.
Devereux, C.P., Holden, T.J., Buchanan, A.H., Pampanin, S., 2011 “NMIT Arts & Media
Building - Damage Mitigation Using Post-tensioned Timber Walls”, Proceedings of
Proceedings of the Ninth Pacific Conference on Earthquake Engineering, “Building an
Earthquake-Resilient Society”, 14-16 April, Auckland, New Zealand, paper 90
Englerkirk, (2002). “Design-Construction of the Paramount – a 39 story Precast Prestressed
Concrete apartment Building”, PCI Journal, Vol. 47, No. 4
Giorgini, S., Taylor, M., Cubrinovski, M., Pampanin, S., 2011 “ Preliminary Observations of
Multi-storey RC Building Foundation Performance in Christchurch following the 22nd
February 2011 Earthquake, NZ Concrete Society Conference, Rotorua
Iqbal, A., Pampanin, S., Buchanan, A.H. and Palermo, A., (2007). “Improved Seismic
Performance of LVL Post-tensioned Walls Coupled with UFP devices”, Proceedings, 8th
Pacific Conference on Earthquake Engineering, Singapore.
Kam, W. Y., Pampanin, S., Palermo, A., and Carr, A. (2006) "Advanced Flag-Shaped
Systems for High Seismic Performance." 1st European Conference in Earthquake
Engineering and Seismology, Geneva, Switzerland.
Kam W.Y., Pampanin S., Palermo A., Carr A. (2010a) Self-centering structural systems with
combination of hysteretic and viscous energy dissipations. Earthquake Engineering &
Structural Dynamics. Aug 2010; 39:10, 1083-1108.
Kam, W. Y., Pampanin, S., Dhakal, R. P., Gavin, H., and Roeder, C. W. (2010b). Seismic
performance of reinforced concrete buildings in the September 2010 Darfield (Canterbury)
earthquakes. Bulletin of the New Zealand Society of Earthquake Engineering, 43:4, 340-350.
Kam, W.Y., Pampanin, S., (2011) General Performance of Buildings in Christchurch CDB
after the 22 Feb 2011 Earthquake: a Contextual Report”, prepared for the Department of
Building and Housing, Department of Civil and Natural Resources Engineering, University
of Canterbury.
Marriott, D., Pampanin, S. , Palermo A., (2009) “Quasi-static and Pseudo-Dynamic testing of
Unbonded Post-tensioned Rocking Bridge Piers with External Replaceable Dissipaters’,
Earthquake Engineering and Structural Dynamics, Vol 38 (3), 331-354
Millen, M., Pampanin, S., Cubrinovki, M.., 2013 “Towards an Integrated Performance-based
Design considering Soil-foundation-structure Interaction”, New Zealand Society for
Earthquake Engineering Conference, Wellington 26-28 April, under preparation
Newcombe, M.P., Pampanin, S., and Buchanan, A.H., (2010a). Design, Fabrication and
Assembly of a Two-Storey Post-Tensioned Timber Building. Proceedings 11th World
Conference on Timber Engineering, Riva del Garda, Italy, 2010.
NZCS, 2010 “PRESSS Deign Handbook” (Editor: S. Pampanin), New Zealand Concrete
Society, Wellington, March
NZS 3101 (2006). "Appendix B: special provisions for the seismic design of ductile jointed
precast concrete structural systems" Standards New Zealand, Wellington.
Palermo, A., Pampanin, S., Buchanan, A.H., and Newcombe, M.P., (2005a). "Seismic Design
of Multi-storey Buildings using Laminated Veneer Lumber (LVL)," Proceedings of the New
Zealand Society for Earthquake Engineering Conference, Wairakei.
Palermo, A., Pampanin, S., Buchanan, A.H., and Newcombe, M.P., (2005a). "Seismic Design
of Multi-storey Buildings using Laminated Veneer Lumber (LVL)," Proceedings of the New
Zealand Society for Earthquake Engineering Conference, Wairakei.
Palermo, A., Pampanin, S. and Carr, A.J., (2005b). Efficiency of Simplified Alternative
Modelling Approaches to Predict the Seismic Response of Precast Concrete Hybrid Systems.
fib symposium, Budapest.
Pampanin, S., Pagani, C., Zambelli, S., (2004), “Cable Stayed and Suspended Solution for
Precast Concrete Frames: the Brooklyn System”, Proceedings of the New Zealand Concrete
Industry Conference, Queenstown, New Zealand, September 16-18.
Pampanin S., (2005). Emerging Solutions for High Seismic Performance of Precast Prestressed Concrete Buildings, Journal of Advanced Concrete Technology (ACT), invited
paper, Vol. 3 (2), pp. 202-222.
Pampanin, S., Palermo, A., Buchanan, A., Fragiacomo, M., Deam, B. 2006 “Code Provisions
for Seismic Design of Multi-Storey Post-tensioned Timber Buildings, CIB Workshop,
Florence, August.
Pampanin, (2010) “Damage-control self-centering structures: from laboratory testing to onsite applications”, Series “Geotechnical, Geological, and Earthquake Engineering, Volume
13”; Chapter 28 in “Advancements in Performance-Based Earthquake Engineering (M Fardis
Editor) Publisher Springer, Part 3, pp. 297-308
Pampanin, S., Kam, W. Y., Tasligedik, A. S., Quintana-Gallo, P., and Akguzel, U.
(2010)"Considerations on the seismic performance of pre-1970s RC buildings in the
Christchurch CBD during the 4th Sept 2010 Canterbury earthquake: was that really a big
one?" Proceedings of the 9th Pacific Conference on Earthquake Engineering, New Zealand
Society for Earthquake Eng. (NZSEE), Auckland.
Pampanin, S., Kam W., Haverland, G., Gardiner, S., (2011) “Expectation Meets Reality:
Seismic Performance of Post-Tensioned Precast Concrete Southern Cross Endoscopy
Building During the 22nd Feb 2011 Christchurch Earthquake” New Zealand Concrete
Industry Conference, Rotorua, August
Pampanin, S., Kam, W. Y., Akguzel, U., Tasligedik, A. S., and Quintana-Gallo, P. (2012).
Report on the observed earthquake damage of reinforced concrete buildings in the
Christchurch CBD on the 22 February 2011 Earthquake. Volume 1 and 2., University of
Canterbury, Christchurch, N.Z.
Park, (2002) Seismic Design and Construction of Precast Concrete Buildings in New Zealand
PCI Journal, Vol. 47, No. 5, pp. 60-75
Paulay, T. and Priestley, M.J.N., (1992). Seismic Design of Reinforced Concrete and
Masonry Buildings. John Wiley & Sons, Chichester, UK.
Priestley, M.J.N. (1991). Overview of the PRESSS Research Programme, PCI Journal,
Vol.36, No.4, pp.50 57.
Priestley, (1996), The PRESSS Program Current Status and Proposed Plans for Phase III, PCI
Journal, Vol. 41, No. 2, pp. 22-40
Priestley, M.J.N. (1998) "Displacement-Based Approaches to Rational Limit States Design of
New Structures", Keynote Address, 11th European Conference on Earthquake Engineering,
Paris, France.
Priestley, M. J. N., Sritharan, S., Conley, J. R. and Pampanin, S. (1999). "Preliminary results
and conclusions from the PRESSS five-story precast concrete test building." PCI Journal,
44(6), 42-67.
Priestley M.J.N., Calvi G.M., Kowalsky M.J. (2007) Displacement-based seismic design of
structures. IUSS Press, Pavia, Italy.
SEAOC, (1995). Vision 2000 Committee, Performance-Based Seismic Engineering,
Structural Engineers Association of California, Sacramento, California.
Smith, T. , Wong, R., Newcombe, M., Carradine, D. , Pampanin, S., Buchanan, A., Seville,
R., McGregor, E., 2011 “The Demountability, Relocation and Re-use of a High Performance
Timber Building”, Proceedings of the Ninth Pacific Conference on Earthquake Engineering,
“Building an Earthquake-Resilient Society”, 14-16 April, Auckland, New Zealand, paper 187
Stanton, J. F., Stone, W. C., Cheok, G. S. (1997), “A Hybrid Reinforced Precast Frame for
Seismic Regions”, PCI Journal, Vol. 42, No. 2, pp. 20-32.
Tasligedik, A.S. , Pampanin, S. , Palermo, A., 2011 “Damage Mitigation Strategies of ‘NonStructural’ Infill Walls: Concept and Numerical-Experimental Validation Program,
Proceedings of the Ninth Pacific Conference on Earthquake Engineering, “Building an
Earthquake-Resilient Society”, 14-16 April, Auckland, New Zealand, paper 120